“Graphene” is a single layer of carbon atoms in a two-dimensional honeycomb array. This material has been studied intently during the past few years, largely because of its unique property as a ballistic electron conductor. Most of the interest in this field has been focused on developing graphene-based electronics, although other potential applications for graphene and graphene hybrid/composite materials include thermal transport, battery electrode materials, catalyst support, hydrogen storage, etc. Graphene-based electronics should theoretically overcome inherent limitations of state-of-the-art silicon-based electronics. A reliable method for the preparation of graphene layers on electronically-relevant materials is a critical part of development of graphene-based electronics.
Silicon carbide (SiC) is a well-known material for good hardness and chemical stability, and has been pursued for many applications at high power, high frequency, high voltage, and high temperature. It is highly desirable to develop a convenient and stable method to put graphene layers on SiC, among other materials.
Four methods have been used to obtain graphene samples with varying degrees of success. They are chemical exfoliation, mechanical exfoliation, thermal vapor process, and the use of polycyclic aromatic hydrocarbons.
(1) Chemical exfoliation is a solution-based process in which graphite is first oxidized to produce hydroxyl, carboxyl, and epoxide groups on individual graphene sheets so that they then can be exfoliated easily in solution. The graphite oxide product subsequently needs to be reduced and transformed into graphene. However, it has not been possible to remove all the functional groups from graphite oxide, and the remaining oxygen-containing groups and lattice defects degrade the electrical properties of graphite oxide derived graphene compared to pristine graphene. In addition, it is difficult to transfer these graphene sheets onto substrates of interest. Regarding prior art documents, note is made of the following:
a) Ruoff et al., “Preparation and characterization of graphene oxide paper” Nature 2007, 448, 457.
b) X Li, G Zhang, X Bai, X Sun, X Wang, E Wang, and H Dai, “Highly conducting graphene sheets and Langmuir-Blodgett films” Nature Nanotechnology, 2008, 3, 538.
c) X Fan, W Peng, Y Li, X Li, S Wang, G Zhang, and F Zhang, “Deoxygenation of Exfoliated Graphite Oxide under Alkaline Conditions: A Green Route to Graphene Preparation” Adv. Mater. 2008, 20, 4490.
d) G Eda, G Fanchini, and M Chhowalla, “Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material” Nature Nanotechnology, 2008, 3, 270.
e) Y Hernandez, et al. “High-yield production of graphene by liquid-phase exfoliation of graphite” Nature Nanotechnology, 2008, 3, 563.
f) G Wang, J Yang, J Park, X Gou, B Wang, H Liu, and J Yao, “Facile Synthesis and Characterization of Graphene Nanosheets” J. Phys. Chem. C 2008, 112, 8192.
g) D Li, et al. “Processable aqueous dispersions of graphene nanosheets” Nature Nanotechnology, 2008, 3, 101.
(2) Mechanical exfoliation removes graphene monolayers from bulk graphite crystals by scraping or a “scotch tape” technique and deposits them onto substrates. This is a highly empirical, low-yielding, and time-consuming process, and again, it is difficult to couple the graphene sheets to the substrate. Regarding the prior art, note is made of the following document:
h) Zhang, et al., “Landau-Level Splitting in Graphene in High Magnetic Fields” Phys. Rev. Lett. 2006, 96, 136806.
(3) An annealing process at elevated temperature under vacuum depletes the Si atoms from a SiC surface, resulting in the formation of graphene from all the residual carbon atoms. Graphene from this approach is closely associated with the single crystal substrate, and results have indicated that graphene's electronic properties can change significantly when coupled in this manner compared to isolated graphene sheets (for better or for worse). Like mechanical exfoliation, this method can provide only relatively small graphene samples for fundamental studies, not enough for larger scale applications.
i) Forbeaux, et al., “Heteroepitaxial graphite on 6H—SiC (0001): Interface formation through conduction-band electronic structure” Phys. Rev. B 1998, 58(24) 16396.
(4) Polyphenylenes or alkyl-functionalized polycyclic aromatic hydrocarbons have been deposited on Cu(110) or quartz surfaces, respectively, followed by thermally-induced cyclodehydrogenation to form graphene islands or domains of about 5-10 nm.
j) Beernink, G., et al. “Synthesis of Polycyclic Aromatic Hydrocarbons and Graphite Islands via Surface-Induced Reaction of Small Molecules” ChemPhysChem 2001, 317.
k) C D Simpson, J D Brand, A J Berresheim, L Przybilla, H J Räder, and K M{umlaut over (υ)}llen, “Synthesis of a Giant 222 Carbon Graphite Sheet” Chem. Eur. J. 2002, 8, 1424.
l) L Zhi and K Müllen, “A bottom-up approach from molecular nanographenes to unconventional carbon materials” J. Mater. Chem. 2008, 18, 1472.
m) X Wang, L Zhi, N Tsao, Z Tomovi{hacek over (c)}, J Li, and K Müllen, “Transparent Carbon Films as Electrodes in Organic Solar Cells” Angew. Chem. Int. Ed. 2008, 47, 2990.
Preliminary indications that the interactions between graphene and silicon carbide substrates can lead to the opening of a semiconductor gap have been published recently. In addition, several computational studies find that the chemical functionalization of graphene should lead to bandgap opening. See:
n) S Y Zhou, et al. “Substrate-induced bandgap opening in epitaxial graphene”Nature Materials 2007, 6, 770.
o) F OuYang, et al. “Chemical Functionalization of Graphene Nanoribbons by Carboxyl Groups on Stone-Wales Defects” J. Phys. Chem. C 2008, 112, 12003.
p) D W Boukhvalov and M I Katsnelson, “Chemical Functionalization of Graphene with Defects: Nano Letters 2008, 8, 4373.